The general art of power steering systems is known but the particular adaptation of specific controls responsive to selected ones of various quantities such as speed signals is not known to the extent of applying it a kind of parking power steering system of the specific arrangements herein disclosed in this new and improved arrangement.
Power steering means which function in accordance with the so-called revolving principle are very widespread. These very sensitive power steering means operate over the entire speed range. They have a hydraulic circuit of their own, which has recently, however, also been used together with a reservoir charging valve and a hydraulic reservoir for operating a hydraulic brake force amplifier. In the brake force amplifier itself, there is then a separation of the media of the hydraulic circuits. In the brake circuit itself, a fluid which meets the requirements of the brake system is used. There are also isolated examples of vehicles which have a so-called central hydraulic system, in which one medium is used both for the power steering and the regulation of the level, as well as for the brakes. In this case, however, the brake circuit is embodied not as a closed circuit but rather as open or even dynamic brake circuit. In a conception of this kind, the fluid must be able to meet strict requirements pertaining to lubricating capacity, because the hydraulic pump runs continuously and supplies the system noted above.
The types of power steering systems which have been known until the present invention usually have a degree of reinforcement which is independent of speed, which is well-known to be disadvantageous. Recently, both in the literature (German Offenlegungsschrift No. 2,352,018) and in isolated production instances, control devices in power steering apparatus have become known which reduce the degree of reinforcement at higher speeds or even cause the power steering mechanism to be effective only at parking speeds. Usually, the engine rpm is used as an auxiliary value for the control means. Power steering means are also known, which are effective only at parking speeds, but whose servo force is derived from an electromotor. These "electromotor power steering" means also use auxiliary signals which restrict the functional range of the power steering. For instance, relay circuits or even electronic circuits are used which control the rotary direction of the engine and shut off the function above a predetermined speed. In some instances, the gear setting or an appropriate final switch is used as a transducer. In consideration of the tendency in automobile design to reduce the weight of the vehicles in the future, which makes the introduction of parking power steering necessary in response to rising comfort standards, still the above-noted means of providing power steering mechanisms require relatively great structural expense and increase the weight of the vehicle.